1. Field of the Invention
The present invention relates to a new and improved heat exchange system for a flash boiler or steam generator and to a method for improving the efficiency and heat recovery of a flash boiler; and, more particularly, to a new and improved coil bank for a steam generator.
2. Description of the Background Art
Typically, steam generators or flash boilers of the type illustrated in U.S. Pat. No. 2,735,410 hereby incorporated by reference and include generating systems and coil banks of the type illustrated in U.S. Pat. No. 2,160,644, also incorporated by reference. These systems and coil banks operate on the assumption that the coldest fluid being heated should be in contact with the hottest flue gas. These units use "straight through" coil flow to generate steam from a group of serial connected helical coils that define a heat exchanger. Water under pressure is introduced at one end of one coil, heated throughout the length of the interconnected coils and delivered as steam at the outlet. These systems and coil banks are generally designated "counter flow" units in that water at low temperature is introduced in the coil farthest from the source of heat energy at the point where combustion gases are at the lowest temperature.
To force fluid through the heat exchanger, a pressure source is necessary. The pressure source must be of a pressure equal to the steam outlet pressure plus the pressure drop in the heat exchanger. Pressure drop in the heat exchanger is substantial since resistance to flow during fluid phase change is high.
Due to the very large and variable heat absorption of fluid in phase change or boiling, actual temperature profiles of these heat exchangers demonstrate that with the present coil system, the highest fluid temperature occurs near the center of the flow path through the coils. Fluid temperature then drops as the fluid in phase change absorbs more heat. Fluid temperature is highest at this point because the fluid is heated without phase change. As fluid saturation temperature is reached, fluid temperature drops due to boiling. In the present heat exchanger, the point of highest fluid temperature establishes a fluid temperature distribution such that the mean fluid temperature is relatively low in relation to the flue gas heat source. This reduces the efficiency of the heat exchanger.
Further, diameters of these heat exchangers are not matched to fluid boiling locations. Since steam is of a significantly larger volume than liquid, the poor cross sectional match exacerbates pressure drop through the coils.
In heat exchangers of this type, the coils are wound in a counter helix configuration and held together by a supporting structure including block and sinuous spacers to define open spaces for gas flow around the coils. An example of this structure is illustrated in U.S. Pat. No. 3,720,259. For these coils, due to the reduction in flue gas temperature, the heat absorption capability of the outer layers is less than the inside layers. It is desirable to increase the absorption capability of the outside of the coils without increasing the space taken by the coils and not appreciably increasing flue gas pressure drop around the coils. Extended surfaces on the coils have been attempted but these surfaces tend to interlock obstructing the spaces between the coils. However, the structure disclosed provides extended surfaces which do not interlock.